Gastrointestinal tract sphincters

There are a number of side-effects of opiates that are due to their actions on opiate receptors outside the central nervous system. Opiates constrict the pupils by acting on the oculomotor nucleus and cause constipation by activating a maintained contraction of the smooth muscle of the gut which reduces motility. This diminished propulsion coupled with opiates reducing secretion in the gut underlie the anti-diarrhoeal effect. Opiates contract sphincters throughout the gastrointestinal tract. Although these effects are predominantly peripheral in origin there are central contributions as well. Morphine can also release histamine from mast cells and this can produce irritation and broncho-spasm in extreme cases. Opiates have minimal cardiovascular effects at therapeutic doses. 

Effect on smooth muscles Acetylcholine causes increase in tone, amplitude of contractions, peristalsis and secretory activity of the gastrointestinal tract. It causes contraction of smooth muscles of gall bladder and relaxation of sphincters of gastrointestinal and biliary tract. 

The gastrointestinal tract is the only system outside the central nervous system (CNS) with significant concentrations of opioid receptors. This reflects their common embryonic origins. Opioids increase intestinal tone and decrease propulsive peristalsis, resulting in delayed gastric emptying and constipation or ileus. Opioids increase common bile duct pressure and decrease bile production and flow, primarily because of spasm of the sphincter of Oddi. The tone of the bile duct itself is also increased. 

Other actions In the gastrointestinal tract, acetylcholine increases salivary secretion, and stimulates intestinal secretions and motility. Bronchiolar secretions are also stimulated. In the genitourinary tract, the tone of the detrusor urinae muscle is increased. In the eye, acetylcholine is involved in stimulating ciliary muscle contraction for near vision and in the constriction of the pupillae sphincter muscle, causing miosis (marked constriction of the pupil). 

CCK influences gastric motility, movements of the gastrointestinal tract that influence the passage of food. CCK stimulates contraction of the pyloric sphincter or pylorus. A sphincter is a ring of muscles that controls the rate of passage of material through an opening or channel in the body. The pylorus is located at the... 

They also increase sphincter tone and decrease secretory activity along the gastrointestinal tract. Decreased motility enhances fluid and electrolyte reabsorption and decreases the volume of intestinal contents. 

Domperidone is a dopamine D2-receptor antagonist with prokinetic and antiemetic properties. It acts primarily on dopamine receptors in the gastrointestinal tract to enhance gastric and oesophageal sphincter tone, gastric emptying and propulsion of intestinal contents. 

Morphine decreases the propulsive contractions of the gastrointestinal tract, and biliary and pancreatic secretions are reduced. The end result, especially when morphine is administered over extended time periods, is constipation. Morphine-induced spasms of the sphincter of Oddi have been observed. However, the clinical significance of such an occurrence should be assessed on an individual basis. Although morphine s effect on the urinary bladder varies, urinary... 

The exact site of absorption of strontium in the gastrointestinal tract is not known however, studies in hamsters suggest the possibility of absorption in both the stomach and small intestine. In hamsters that received a gavage tracer dose of 85SrCl2, 37% was absorbed, whereas 20% was absorbed when the dose was administered to hamsters that had their pyloric sphincter ligated (Cuddihy and Ozog 1973). Studies in preparations of in vitro and in situ isolated intestine of the rat provide direct evidence for strontium absorption in the small intestine (see Section 3.5.1). 

Morphine reduces the activity of the entire gastrointestinal tract in that it reduces the secretion of hydrochloric acid, diminishes the motility of the stomach, and increases the tone of the upper part of the duodenum. These actions may delay passage of the stomach contents into the duodenum. Both pancreatic and biliary secretions are diminished, and this may also hinder digestion, hi the large intestine, the propulsive peristaltic wave in the colon is reduced, the muscle tone including that of the anal sphincter is increased, and the gastrocohc reflex (defecation reflex) is reduced. These actions, in combination, cause constipation, which seems to be a chronic problem among addicts. 

Sinoatrial node Atrial contractility Bronchiole smooth muscle Gastrointestinal tract Smooth muscle Secretions Sphincters... 

CNS confusion, faintness, dysphoria, hallucinations, unusual dreams, restlessness cardiovascular hypotension, bradycardia, hypertension, tachycardia gastrointestinal dyspepsia, cramps, nausea, vomiting, biliary tract sphincter spasm respiratory depression... 

Nonneurogenic bladder-sphincter dysfunction is thought to originate from behavioral factors that affect toilet training and inhibit the maturation of normal urinary control. Since the gastrointestinal tract plays a prominent role in lower urinary tract dysfunction, the term dysfunctional elimination syndromes (Koff et al.l998) is applied, if functional bowel disturbances are associated in terms of chronic constipation and encopresis. 

Other essential effects of morphine are the elevation of muscle tone in the gastrointestinal tract due to the reduction of acetylcholine liberated from the parasympathetic nerve endings, thus leading to a depression of peristalsis and a reduction in the stretching of the g.i. tract. As a result there is a reduction in stomach emptying and defaecation. The tone of the gall bladder and also of the sphincter muscle of the urinary bladder is also enhanced. 

Caffeine relaxes smooth muscle of the biliary and gastrointestinal tracts and has a weak effect on peristalsis. However, high doses can produce biphasic responses, with an initial contraction followed by relaxation. Caffeine seems to have no effect on the lower oesophageal sphincter. The increase in both gastric and pepsin secretions is linearly related to the plasma levels obtained after the administration of a dose of 4-8mgkg . In the small intestine, caffeine modifies the fluid exchange from a net absorption to a net excretion of water and sodium. 

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